Formula to calculate output power of class A amp?

[SiliconWizard]

2W is a lot for headphones. But, but. As a very general consideration, headphones with a higher impedance (such as your 300 ohm figure) absolutely do not need that much power anyway. Possibly, *some* "high-grade" headphones, with low impedance and a relatively low efficiency (low dB SPL/W @ xxx m), can require 1W to 2W RMS to get loud, but those are really niche devices.

Trying to design a "universal" headphones amp that can accomodate such a wide impedance range (say from 4 ohm to 300 ohm) is not necessarily a good idea. Just a thought.

[NiHaoMike]

The easiest solution is to just use a chip specifically designed to drive headphones, such as the TPA6120A2.

[DimitriP]

2 watt for a headphone? Watt for? sorry the pun.
https://nwavguy.blogspot.com/2011/09/more-power.html

Might be a class project, one of those "show your work" type projects and by using a semi-unrealistic value it may supposedly make it harder to find something to copy.
But I just made all this up. I could be waaaay off. Or not!

 

You are way off indeed. Why don't you go somewhere else to troll and spam the chat if you don't have any answers to the question Dimitri? @DimitriP

Come to think of it, I wasn't "talking to you" in the first place. So excuse you.
2W headphone amp without any reasoning invites speculation.

Sometimes you get the answers you need, not the answers you want.
 

[CaptDon]

Your single transistor solution to a 2 watt class A amplifier is bullocks and certainly not done with a 2n3904!!! A class A amplifier nearly mandates the output stage dissipates 1/2 of the total output power at rest. Calculate the peak to peak voltage required to get 2 watts across 300 ohms. Your output power supply voltage will have to be at least that amount plus about 10 to 15% to prevent clipping. With the coupling capacitor you can use a single sided power supply even with most any off the shelf output integrated circuit that is typically class B push pull topology. You don't need Class A AudioPhool design. Just buy off the shelf audio output I.C.'s for you project. Some don't even need external support devices, only an input level control and power.

[Leuams]

Douglas Self has a few great books on audio amplifier design if you wish to go down that rabbit hole. I also found sound-au.com as a great source.

[7AudioBB]

2W is a lot for headphones. But, but. As a very general consideration, headphones with a higher impedance (such as your 300 ohm figure) absolutely do not need that much power anyway. Possibly, *some* "high-grade" headphones, with low impedance and a relatively low efficiency (low dB SPL/W @ xxx m), can require 1W to 2W RMS to get loud, but those are really niche devices.

Trying to design a "universal" headphones amp that can accomodate such a wide impedance range (say from 4 ohm to 300 ohm) is not necessarily a good idea. Just a thought.

My initial goal was to cover all kinds of headphones but I agree, it might not be a good or even a safe option. Thanks for your reply.

[7AudioBB]

The easiest solution is to just use a chip specifically designed to drive headphones, such as the TPA6120A2.

Thanks, but then I guess I would miss out on the fun of learning, designing and building a class a circuit.

[BrianHG]

You cannot create a Class A audio amp designed to drive a 300ohm headphone at 2 watts and expect it to be an appropriate design to drive at standard 32ohm headphone at the same time.

Your pull-down resistors will be the wrong value by an order of magnitude and your power supply will be bonkers at 120v DC.

If you need class A, you will have to either design for the 16ohm to 32ohm headphones, or a completely different design for 300ohm headphones.

And a warning about having 2 watt capabilities for 32 ohm headphones, driving a 16 ohm headphones means you will be delivering up to 6 watts into each ear (depending on design).  Someone will sue if you blow their expensive headphones let alone melting their ears.

Remember, for 2 watts per channel, at 32 ohms, you will need ~+/-9v supply if you have a rail-to-rail output driving amp.
For 2 watts into 16 ohms, you will only need ~+/-6v.

For simple class A, you will need to double the negative voltage for the pull-down resistor unless you use an active constant current sink for your pull-down like the high end Pass Labs Class A amplifiers.  However, some argue that this additional transistor element on the output counts as a 'gain' element, but you efficiency goes up quite a bit, almost 25%, as your power supply needs drop by 25%.  This means only needing +/-9v for this class A amp driving 2 watts into a 32 ohm headphone load just like a Class-AB or Class-D amp.  (You might need an extra 3-4v for the constant current source to begin operation at the bottom end, but not double your source voltage.  And the current buffer gain/load curve is will also be linear...)

[7AudioBB]

You cannot create a Class A audio amp designed to drive a 300ohm headphone at 2 watts and expect it to be an appropriate design to drive at standard 32ohm headphone at the same time.

Your pull-down resistors will be the wrong value by an order of magnitude and your power supply will be bonkers at 120v DC.

If you need class A, you will have to either design for the 16ohm to 32ohm headphones, or a completely different design for 300ohm headphones.

And a warning about having 2 watt capabilities for 32 ohm headphones, driving a 16 ohm headphones means you will be delivering up to 6 watts into each ear (depending on design).  Someone will sue if you blow their expensive headphones let alone melting their ears.

Remember, for 2 watts per channel, at 32 ohms, you will need ~+/-9v supply if you have a rail-to-rail output driving amp.
For 2 watts into 16 ohms, you will only need ~+/-6v.

For simple class A, you will need to double the negative voltage for the pull-down resistor unless you use an active constant current sink for your pull-down like the high end Pass Labs Class A amplifiers.  However, some argue that this additional transistor element on the output counts as a 'gain' element, but you efficiency goes up quite a bit, almost 25%, as your power supply needs drop by 25%.  This means only needing +/-9v for this class A amp driving 2 watts into a 32 ohm headphone load just like a Class-AB or Class-D amp.  (You might need an extra 3-4v for the constant current source to begin operation at the bottom end, but not double your source voltage.  And the current buffer gain/load curve is will also be linear...)

Thank you for your insights Brian. I'm going to consider this and give the design a narrower target range in terms of headphone impedance.

[7AudioBB]

Douglas Self has a few great books on audio amplifier design if you wish to go down that rabbit hole. I also found sound-au.com as a great source.

Thank you @Leuams, amplifiers seem to be a good entry point for me to learning circuit design. I'm gonna check those sources out!